The present invention relates generally to inverter configurations, such as may be used for an advanced static VAR (volt-amperes-reactive) compensator (ASVC) for power systems, and more particularly to an apparatus and method for providing quasi-harmonic neutralization in voltage sourced static inverter systems by combining outputs from multiple sets of phase displaced inverter outputs. Other uses for these inverter systems currently include airborne power systems, space stations, power supply inverters, generation of controllable leading or lagging reactive current, industrial drives, various military applications requiring high quality power, power conditioning, and DC link frequency conversion.
In such applications, three phase static inverters typically employ semiconductor switching devices arranged in a 6-pulse bridge configuration consisting of three inverter poles connected across a DC voltage source. In low power applications, high quality sinusoidal output waveforms are obtained using fast switching transistors with high frequency switching or pulse width modulation techniques. In higher power applications, such high frequency switching becomes less efficient, so high quality output voltage waveforms are obtained using programmed waveform techniques with a reduced number of switchings in each fundamental period.
For example, conventional harmonic neutralized inverters, such as those used in advanced static VAR generators, employ many large semiconductor switches and a high efficiency, so the switches are operated at fundamental frequency using special transformer configurations for harmonic neutralization. The outputs from a number of inverter stages, each operating at fundamental frequency, are combined to produce a high quality multi-step output waveform. This output waveform has each step evenly spaced with an amplitude proportional to the sine of the angular position. The number of steps in the output voltage waveform is referred to as the "pulse number." In the typical multi-step or multi-pulse inverter, each switching device contributes equally to the output while operating with identical voltage and current waveforms. Since each switch turns on and off at the same current level and operates with similar delays, the effect upon the output waveform of differences in current-dependent switching delays are minimized.
The 6-pulse bridge, which is simplest of the three phase harmonic neutralized inverters, has three inverter poles, each having two switching devices connected in series with the junction of the switches being the AC output terminal. Each inverter pole operates at fundamental frequency, so three square wave outputs are produced with respect to the midpoint of the DC voltage. These three square wave outputs are symmetrically displaced by 120.degree.. Thus, every 60.degree. a pole transition occurs so there are six state changes in a single cycle of the fundamental frequency. At the three AC terminals, the output voltages are true 6-pulse waveforms. The basic 6-pulse bridge inverter requires no transformer and is generally used as the basic building block to construct higher pulse number inverters.
True harmonic neutralized outputs having a pulse number of (N.times.6) are produced by combining the outputs of N 6-pulse bridges, with each of the 6-pulse bridges being coupled to a common DC source. Each bridge output is incrementally phase displaced from the preceding output by an angle of 360.degree./6N. To shift the fundamental outputs of individual 6-pulse bridges into phase with one another, special individual transformers having appropriate winding configurations and identical voltage ratios are required. The transformed outputs are combined, either by placing all of the transformer secondaries in series (in inverter terminology, the "secondaries" are the windings on the AC output side of the transformer, with the primary windings being coupled to the inverter), by parallel connection through appropriate additional interphase transformers, or by some combination of these series and parallel connection schemes.
To derive the necessary phase shifts to combine the output waveforms, the individual transformers must have differently configured windings for each 6-pulse building block. In practical transformers, integer turns must be used to obtain the required phase shifts. This imposes limitations on the desired ratios which may be obtained, so accurate incremental phase shifts may not be possible with transformers having a low number of winding turns. For example, a 72-pulse system needs twelve different transformers, each having a unique turns ratio and configuration, to provide true harmonic neutralization. Although significant improvement in waveform quality is obtainable using higher pulse numbers, the increased complexity and cost of the special transformers cannot be justified for most applications. So typically, the tradeoff between waveform quality and initial cost is weighted toward simple winding configurations and low pulse numbers. The highest pulse number typically employed is twelve, using special transformers with combinations of wye and delta windings.
In the past, to obtain high quality AC output waveforms and high efficiency from large inverters, it has been traditional to employ harmonic neutralizing techniques. As described above, one typical harmonic neutralizing technique combines the outputs from a number of individual 6-pulse bridges or inverter stages operating at a fundamental frequency using a multitude of different phase-shifting transformers. Such an arrangement allows the fundamental components of the output voltages to sum directly, while the undesirable and dominant harmonic components cancel out one another. However, to obtain an output waveform of acceptable quality for most power system utility applications, without adding harmonic filters, a pulse number of at least thirty-six is required.
For example, a classical 36-pulse inverter has six 6-pulse bridges and six corresponding specially designed different phase-shifting transformers. Each inverter stage is displaced from the preceding stage by 10.degree. (=360.degree.+36, where 36=6N=6.times.6). Alternatively, the same effect is achieved with more complex transformer structures, such as transformers having multiple windows. One significant drawback to each of these earlier approaches is the initial equipment expense, as well as the high operating cost incurred in heating (I.sup.2 R) losses and the like.
Several simplified configurations for multi-pulsed voltage sourced inverters have recently evolved. These configurations employ different techniques to produce quasi-harmonic neutralized output waveforms using only one or two three phase transformers for 24-pulse outputs. One example of such an arrangement is disclosed in U.S. Pat. No. 4,870,557 to Eric J. Stacey, an inventor of the invention illustrated below.
Thus, a need exists for an improved high power voltage sourced inverter system, such as may be used to provide improved quasi-harmonic neutralization in voltage sourced static inverter systems, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.